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Re: Split Parabola Boundary Array

Subject: Re: Split Parabola Boundary Array
From: "Rob Danielson" danielson_audio
Date: Fri Jan 25, 2008 9:45 am ((PST))
At 6:49 AM +0000 1/24/08, John Hartog wrote:
>  > Perhaps from the dish cavity itself. A little dampening of the
>>  surfaces of the center divider might eliminate it.
>Hi Rob,
>I taped a couple foam disks (6 x 1/2") to each side of the barrier,
>and tried it out recording some stream sounds. It is still hard to
>tell if it helped at all - the nicest and worst sounding recordings of
>the session had more to do with my choice of direction than anything else.

Thanks for reporting back, John. If both sides of the barrier are 
fully-covered, this should reduce reflections from the barrier back 
into the dish. I've used 1/2" thick, heavy foam padding for sound 
deadening. There are some two-layer mat products that use open cell 
foam on the surface and high density foam behind. The resonance is 
lower in the spectrum, so higher density and thicker might work best. 
The frequencies in running water might be too high to test its 
effectiveness. You could direct the dish to a neutral spot on the 
horizon in a quiet location for the right frequency content. Record 
for several minutes with and without the foam and a fixed dish 
position. The foam shouldn't have a negative impact; the test is only 
for the curious.

>One thing I noticed while out testing it last weekend is the position
>of the capsules relative to the parabolic focus can make a big
>difference in the quality of the sound. Pulling the capsules deeper
>into the dish improves the overall balance of qualities in some instances
>To understand this a bit more I am using a laser pointer to study dish
>reflections. I see a general pattern in how a parabola reflects
>off-axis sounds towards the barrier. Wider off-axis angles reflect the
>signal deeper toward the back of the dish. But more importantly,
>off-axis signals from the opposite side of the barrier that are not
>blocked by the barrier reflect out beyond the parabolic focus - the
>further off, the further out. To avoid any flip-flop effects from
>that, it might be important to keep mic capsules positioned at or
>behind the parabolic focus, or else far enough out to be beyond those
>opposite-side reflections.

Maybe being able to adjust the depth and orientation of the capsules 
is preferable? They could be mounted on a ball swivel attached to a 
track that slides in and out. Rob D.

>At any rate it will be a while until I try more than two microphones
>in my array, but I am enjoying following the progression of Tom's
>multi-capsule experiments.
>John Hartog


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